Journal of Athletic Training 2002;37(1):71±79 q by the National Athletic Trainers' Association, Inc www.journalofathletictraining.org The Sensorimotor System, Part I: The Physiologic Basis of Functional Joint Stability Bryan L. Riemann; Scott M. Lephart University of Pittsburgh, Pittsburgh, PA Bryan L. Riemann, PhD, ATC, contributed to conception and design; acquisition and analysis and interpretation of the data; and drafting, critical revision, and ®nal approval of the article. Scott M. Lephart, PhD, ATC, contributed to conception and design; analysis and interpretation of the data; and drafting, critical revision, and ®nal approval of the article. Address correspondence to Bryan L. Riemann, PhD, ATC, Georgia Southern University, PO Box 8076, Statesboro, GA 30460± 8076. Address e-mail to [email protected]. Objective: To de®ne the nomenclature and physiologic tal roles in optimal motor control and sensorimotor control over mechanisms responsible for functional joint stability. functional joint stability. Data Sources: Information was drawn from an extensive Conclusions/Applications: Sensorimotor control over the MEDLINE search of the scienti®c literature conducted in the dynamic restraints is a complex process that involves compo- areas of proprioception, neuromuscular control, and mecha- nents traditionally associated with motor control. Recognizing nisms of functional joint stability for the years 1970 through and understanding the complexities involved will facilitate the 1999. An emphasis was placed on de®ning pertinent nomen- continued development and institution of management strate- clature based on the original references. gies based on scienti®c rationales. Data Synthesis: Afferent proprioceptive input is conveyed to Key Words: proprioception, neuromuscular, motor control all levels of the central nervous system. They serve fundamen- he purpose of this 2-part series is to provide an over- our discussion by de®ning some broad terms used in the med- view of the current understanding surrounding periph- ical and physiologic literature. Homeostasis is de®ned as the Teral afferent information acquisition and processing and dynamic process by which an organism maintains and controls levels of motor control as they relate to functional joint sta- its internal environment despite perturbations from external bility. We recognize that these papers focus heavily upon basic forces.1 Because cells, tissues, and organs operating within the science research that, in many circumstances, lacks immediate body can only function within narrow ranges of environmental clinical application. Our premise is to present the athletic train- conditions, maintaining homeostasis becomes the major driv- ing community with an introduction concerning how the dy- ing force underlying many, if not all, physiologic functions of namic restraints are activated and controlled by the motor con- the body. The body is composed of many systems that operate trol system of the body. Our goal is that these papers may automatically and subconsciously to maintain the body in a initiate common understanding regarding the terminology and homeostatic condition.2 A system is speci®cally de®ned as an underlying physiology associated with proprioception and neu- organized grouping of related structures that perform certain romuscular control. Ultimately, by establishing a baseline un- common actions.3 Systems are organized hierarchically, be- derstanding about the sensorimotor system, clinical techniques ginning at the cellular level, and contribute to bodily homeo- can continue to be developed and applied with scienti®c ratio- stasis in speci®c domains. In a healthy individual, the system's nale. Furthermore, through this understanding, clinicians can homeostasis is usually maintained by 2 different control sys- appreciate future developments and research directions focus- tems. Stimulation of a corrective response within the corre- ing on the restoration of functional joint stability. sponding system after sensory detection is often considered The purpose of this ®rst paper is to introduce the sensori- feedback controls. In contrast, feedforward controls have been described as anticipatory actions occurring before the sensory motor motor system, the biological system that controls the 4,5 contributions of the dynamic restraints for functional joint sta- detection of a homeostatic disruption. Initiated feedback ac- bility. A secondary purpose is to de®ne the nomenclature per- tions are largely shaped by previous experience with the de- taining to the mechanisms responsible for both the sensory and tected stimulus. Somatosensory, visual, and vestibular input motor components of proprioception and neuromuscular con- provides the information necessary for both forms of control during motor activities; however, the methods of information trol for the maintenance of functional joint stability. processing differ.5 Feedback control is characterized by a con- tinual processing of afferent information, providing response PERTINENT TERMINOLOGY control on a moment-to-moment basis. In contrast, afferent Before examining the specialized components and physio- information during feedforward control is used intermittently logic intricacies of the sensorimotor system, we must begin until feedback controls are initiated.5,6 Journal of Athletic Training 71 Unfortunately, classifying an action as either feedback or feedforward is not as straightforward as their de®nitions sug- gest. In some circumstances, a combination of both feedfor- ward and feedback control exists, such as during the mainte- nance of postural control.6 Additionally, consider the situation in which a subject watches a tester trigger a device that in- duces a joint perturbation. Many subjects will naturally ``tense up'' when they see the tester beginning to push the trigger before the perturbation. Whether the muscle activation before the perturbation reaching the joint is the result of feedforward or feedback control remains controversial. For this reason, the term feedforward control has been recommended to describe actions occurring upon the identi®cation of the beginning, as well as the effects, of an impending event or stimulus.4,5,7 In contrast, feedback control should be used to describe actions occurring in response to the sensory detection of direct effects from the arrival of the event or stimulus to the system. Figure 1. The sensorimotor system incorporates all the afferent, The actions occurring with both feedback and feedforward efferent, and central integration and processing components in- controls involve the hierarchic organization of a system, be- volved in maintaining functional joint stability. Although visual and ginning at the cellular level and extending through both the vestibular input contributes, the peripheral mechanoreceptors are tissue and organ levels. The action patterns used to restore the most important from a clinical orthopaedic perspective. The homeostasis are de®ned as mechanisms.3 For example, the re- peripheral mechanoreceptors (pictured on the lower left) reside in ¯exive response is the mechanism the body uses to maintain the cutaneous, muscular, joint, and ligamentous tissues. Afferent or restore joint stability after an imposed joint perturbation. pathways (dotted lines) convey input to the 3 levels of motor con- Within a given mechanism are multiple processes that ulti- trol and associated areas such as the cerebellum. Activation of mately lead to the achievement of the result. In the case of motor neurons may occur in direct response to peripheral sensory input (re¯exes) or from descending motor commands, both of joint perturbation, the processes include mechanoreceptor which may be modulated or regulated by the associate areas (gray stimulation, neural transmission, integration of the signals by lines). Efferent pathways from each of the motor control levels the central nervous system (CNS), transmission of an efferent (solid lines) converge upon the alpha and gamma motor neurons signal, muscle activation, and force production. By de®nition, located in the ventral aspects of the spinal cord. The contractions for the purposes of this paper, assessing a mechanism refers by the extrafusal and intrafusal muscle ®bers cause new stimuli to to the cumulative outcome of the underlying processes. During be presented to the peripheral mechanoreceptors. many clinical and research evaluations, inferences about the integrity of mechanisms are made by measuring speci®c char- prise the static (passive) components.10,11 Dynamic contribu- acteristics of the underlying processes. Onset latency of mus- tions arise from feedforward and feedback neuromotor control cle activation, as measured through electromyography, is fre- over the skeletal muscles crossing the joint. Underlying the quently assessed in joint perturbation. effectiveness of the dynamic restraints are the biomechanical One additional physiologic term requiring attention in a and physical characteristics of the joint. These characteristics broad context before our specialized discussion is stability. include range of motion and muscle strength and endurance. Stability is de®ned as the state of remaining unchanged, even From these descriptions of static and dynamic stability com- in the presence of forces that would normally change the state ponents, it becomes apparent that the terms are not synony- 3 It has been further described as the property of or condition. mous. Integrity of static stabilizers is measured through